1) first click "Yes", reboot and check if boot is repaired. (works when your computer is hybrid BIOS & EFI)
2) If still not good, that means that your computer absolutely needs a EFI partition. Please create a EFI partition , then run Boot-Repair again (optionally you can check in the Advanced options that your EFI partition is correctly detected).

dialog changed to:
"The boot of your PC is in EFI mode, but no EFI partition was detected. You may want to retry after creating a EFI partition (FAT32, >200Mo, start of the disk}, EFI flag). Do you want to continue?"